1. Field of the Invention
The present invention relates to a light emitting panel in which a light emitting element formed on a substrate is enclosed between the substrate and a cover member. Also, the present invention relates to a light emitting module in which an IC or the like is mounted on the light emitting panel. Note that, in this specification, the light emitting panel and the light emitting module are generically called light emitting devices. The present invention further relates to a method of driving the light emitting device and an electronic appliance using the light emitting device. Moreover, present invention relates to an element substrate corresponding to one mode before the light emitting element is completed in the step of manufacturing the light emitting device, wherein said element substrate includes means for providing electric current to the light emitting element in a plurality of pixels, respectively.
2. Description of the Related Art
An light emitting element emits light by itself, and thus, has high visibility. The light emitting element does not need a backlight necessary for a liquid crystal display device (LCD), which is suitable for a reduction of a light emitting device in thickness. Also, the light emitting element has no limitation on a viewing angle. Therefore, the light emitting device using the light emitting element has recently been attracting attention as a display device that substitutes for a CRT or the LCD.
Incidentally, the light emitting element means an element of which a luminance is controlled by electric current or voltage in this specification. The light emitting element includes an OLED (organic light emitting diode), an MIM type electron source element (electron emitting elements) used to a FED (field emission display) and the like.
The OLED includes a layer containing an organic compound in which luminescence generated by application of an electric field (electroluminescence) is obtained (organic light emitting material) (hereinafter, referred to as organic light emitting layer), an anode layer and a cathode layer. A light emission in returning to a base state from a singlet excitation state (fluorescence) and a light emission in returning to a base state from a triplet excitation state (phosphorescence) exist as the luminescence in the organic compound. The light emitting device of the present invention may use one or both of the above-described light emissions.
Note that, in this specification, all the layers provided between an anode and a cathode of the OLED are defined as the organic light emitting layers. The organic light emitting layers specifically include a light emitting layer, a hole injecting layer, an electron injecting layer, a hole transporting layer, an electron transporting layer and the like. The OLED basically has a structure in which an anode, a light emitting layer, a cathode are laminated in order. Besides this structure, the OLED may take a structure in which an anode, a hole injecting layer, a light emitting layer, a cathode are laminated in order or a structure in which an anode, a hole injecting layer, a light emitting layer, an electron transporting layer, a cathode are laminated in order. These layers may have an inorganic compound therein.
FIG. 25 shows the structure of a pixel in a general light emitting device. The pixel shown in FIG. 25 has TFTs 50 and 51, a storage capacitor 52, and a light emitting element 53.
A gate of the TFT 50 is connected to a scanning line 55. The TFT 50 has a source and a drain one of which is connected to a signal line 54 and the other of which is connected to a gate of the TFT 51. The TFT 51 has a source connected to a power supply 56 and has a drain connected to an anode of the light emitting element 53. A cathode of the light emitting element 53 is connected to power supply 57. The storage capacitor 52 is provided to hold the voltage between the gate and source of the TFT 51.
When the TFT 50 is turned ON by the voltage of the scanning line 55, a video signal inputted to the signal line 54 is inputted to the gate of the TFT 51. Upon input of the video signal, the gate voltage (voltage difference between the gate and the source) of the TFT 51 is determined in accordance with the voltage of the video signal inputted. The gate voltage causes a drain current to flow in the TFT 51 and the drain current is supplied to the light emitting element 53, which emits light upon receiving the current.
TFTs formed of polysilicon are higher in field effect mobility and larger in ON current than TFTs formed of amorphous silicon. Therefore TFTs formed of polysilicon are more suitable as transistors for light emitting element panels.
However, electric characteristics of polysilicon TFTs are far behind the electric characteristics of MOS transistors that are formed on single crystal silicon substrates. For example, the field effect mobility of polysilicon TFTs is 1/10 or less of the field effect mobility of single crystal silicon TFTs. Furthermore, polysilicon TFTs are easily fluctuated in characteristics because of defects in grain boundaries.
In the case where pixels are structured as shown in FIG. 25, if the threshold, ON current, and other characteristics of the TFT 51 fluctuate from one pixel to another, the amount of drain current of the TFT 51 varies between pixels despite the voltage of the video signal being the same. This leads to fluctuation in luminance of the light emitting element 53.
To avoid the problem described above, various kinds of current input type pixel structures have been devised which can control the amount of current flowing into a light emitting element without being influenced by characteristics of TFTs. Two examples of typical current input type pixel will be given below to describe their structures.
The structure of a current input type pixel disclosed in JP 2001-147659 A is described first with reference to FIG. 26A.
The pixel shown in FIG. 26A has TFTs 11, 12, 13, and 14, a storage capacitor 15, and a light emitting element 16.
A gate of the TFT 11 is connected to a terminal 18. The TFT 11 has a source and a drain one of which is connected to a current source 17 and the other of which is connected to a drain of the TFT 13. A gate of the TFT 12 is connected to a terminal 19. The TFT 12 has a source and a drain one of which is connected to the drain of the TFT 13 and the other of which is connected to a gate of the TFT 13. The TFT 13 and the TFT 14 are connected to each other at their gates. Sources of the TFTs 13 and 14 are both connected to a terminal 20. A drain of the TFT 14 is connected to an anode of the light emitting element 16. A cathode of the light emitting element 16 is connected to a terminal 21. The storage capacitor 15 is provided to hold the voltage between the gate and source of the TFTs 13 and 14. Given voltages are applied to the terminals 20 and 21 from a power supply, and the voltage of the terminal 20 is different from the voltage of the terminal 21.
After the TFTs 11 and 12 are turned ON by the voltages applied to the terminals 18 and 19, the drain current of the TFT 13 is controlled by the current source 17. Since the gate and drain of the TFT 13 are connected to each other, the TFT 13 operates in a saturation region and the drain current of the TFT 13 is expressed by the following Expression 1. VGS represents the gate voltage; μ, the mobility; Co, the gate capacity per unit area; W/L, the ratio of a channel width W of a channel formation region to a channel length L; VTH, the threshold; and I, the drain current.I=μCoW/L(VGS−VTH)2/2  [Expression 1]
In Expression 1, μ, Co, W/L, and VTH are fixed values unique to each transistor. It is understood from Expression 1 that the drain current of the TFT 13 is changed by the gate voltage VGS. According to Expression 1, the level of gate voltage VGS generated in the TFT 13 is determined by the drain current.
At this point, the gate voltage of the TFT 14 is kept at the same level as the gate voltage of the TFT 13 because the gate and source of the TFTs 13 and 14 are connected to each other.
Therefore, the drain current of the TFT 13 and the drain current of the TFT 14 are in proportion to each other. If μ, Co, W/L, and VTH of the TFT 13 are identical with those of the TFT 14, the TFTs 13 and 14 have the same amount of drain current. The drain current flowing in the TFT 14 is supplied to the light emitting element 16, which emits light at a luminance according to the amount of the supplied drain current.
The light emitting element 16 continues to emit light even after the TFTs 11 and 12 are turned OFF by the voltages applied to the terminals 18 and 19, as long as the gate voltage of the TFT 14 is held by the storage capacitor 15.
As described above, the pixel shown in FIG. 26A has means for converting a current supplied to the pixel into a voltage to hold the voltage and means for causing a current to flow to the light emitting element in an amount according to the voltage held. FIG. 27A is a block diagram showing a relation between the means of the pixel of FIG. 26A and the light emitting element of the pixel. A pixel 80 has a converting unit 81 for converting a current supplied to the pixel into a voltage to hold the voltage, a driving unit 82 for causing a current to flow to a light emitting element in an amount according to the voltage held, and a light emitting element 83. The current supplied to the pixel 80 is converted into a voltage in the converting unit 81, and the voltage is given to the driving unit 82. The driving unit 82 supplies a current to the light emitting element 83 in an amount according to the voltage given.
Specifically, the TFT 12, the TFT 13, and the storage capacitor 15 in FIG. 26A correspond to the means for converting a supplied current into a voltage to hold the voltage. The TFT 14 corresponds to the means for causing a current to flow to the light emitting device in an amount according to the voltage held.
Described next with reference to FIG. 26B is the structure of a current input type pixel disclosed in Tech. Digest IEDM 98, 875, by R. M. A. Dawson etc. The pixel shown in FIG. 26B has TFTs 31, 32, 33, and 34, a storage capacitor 35, and a light emitting element 36.
A gate of the TFT 31 is connected to a terminal 38. The TFT 31 has a source and a drain one of which is connected to a current source 37 and the other of which is connected to a source of the TFT 33. A gate of the TFT 34 is connected to the terminal 38. The TFT 34 has a source and a drain one of which is connected to a gate of the TFT 33 and the other of which is connected to a drain of the TFT 33. A gate of the TFT 32 is connected to a terminal 39. The TFT 32 has a source and a drain one of which is connected to a terminal 40 and the other of which is connected to a source of the TFT 33. The drain of the TFT 34 is connected to an anode of the light emitting element 36. A cathode of the light emitting element 36 is connected to a terminal 41. The storage capacitor 35 is provided to hold the voltage between the gate and source of the TFT 33. Given voltages are applied to the terminals 40 and 41 from a power supply and the voltage of the terminal 40 is different from the voltage of the terminal 41.
After the TFTs 31 and 34 are turned ON by the voltage applied to the terminal 38 and the TFT 32 is turned OFF by the voltage applied to the terminal 39, the drain current of the TFT 33 is controlled by the current source 37. Since the gate and drain of the TFT 33 are connected to each other, the TFT 33 operates in a saturation region and the drain current of the TFT 33 is expressed by the above-mentioned Expression 1. It is understood from Expression 1 that the drain current of the TFT 33 is changed by the gate voltage VGS.
According to Expression 1, the level of gate voltage VGS generated in the TFT 33 is determined by the drain current.
The drain current flowing in the TFT 33 is supplied to the light emitting element 36, which emits light at a luminance according to the amount of the supplied drain current.
After the TFTs 31 and 34 are turned OFF by the voltage applied to the terminal 38, the TFT 32 is turned ON by the voltage applied to the terminal 39. The light emitting element 36 continues to emit light at the same luminance as the luminance of light emitted while the TFTs 31 and 34 are ON as long as the gate voltage of the TFT 33 is held by the storage capacitor 35.
As described above, the pixel shown in FIG. 26B has means for converting a current supplied to the pixel into a voltage to hold the voltage and for causing a current to flow to the light emitting element in an amount according to the voltage held. In short, the functions of the two means of the pixel in FIG. 26A are home by one means in the pixel of FIG. 26B. FIG. 27B is a block diagram showing a relation between the means of the pixel of FIG. 26B and the light emitting element of the pixel. In FIG. 27B, one means carries out the function of the converting unit and the function of the driving unit; a current supplied to a pixel 85 is converted into a voltage in means 86 that is a converting unit and at the same time a driving unit, and then the means supplies a current to a light emitting element 87 in an amount according to the voltage converted.
Specifically, the TFT 33, the TFT 34, and the storage capacitor 35 in FIG. 26B correspond to the means for converting a current supplied to the pixel into a voltage to hold the voltage and for causing a current to flow to the light emitting element in an amount according to the voltage held.
When pixels are structured as shown in FIG. 26A or 26B, the amount of current flowing into a light emitting element can be controlled by a current source even if TFT characteristics such as threshold and ON current fluctuate between pixels. Therefore it is possible to prevent fluctuation in luminance of light emitting element between pixels.
In general, lowering of luminance due to degradation of an organic light emitting material is smaller in a light emitting element that emits light with the current between electrodes kept constant than in a light emitting element that emits light with the voltage between electrodes kept constant. In the case of the two current input type pixels illustrated in FIGS. 26A and 26B, the amount of current flowing into a light emitting element can always be kept at a desired value without being influenced by degradation of an organic light emitting material. Accordingly, lowering of luminance due to degradation of organic light emitting element in the pixels of FIGS. 26A and 26B is smaller than in the voltage input type pixel of FIG. 25 where the TFT 51 operates in a linear range.
The luminance of light emitting element and the amount of current flowing in an organic light emitting layer are in proportion to each other. In a current input type light emitting device, the amount of current flowing into a light emitting element can be kept constant regardless of change in organic light emitting element temperature by the outside temperature and by heat generated from the light emitting panel itself. This type of light emitting device therefore can reduce change in luminance of light emitting element and can prevent an increase in current consumption due to a temperature rise.
However, the two pixels of FIGS. 26A and 26B also have problems.
In a pixel that has two means, one for converting a current supplied to the pixel into a voltage to hold the voltage and one for causing a current to flow to a light emitting element in an amount according to the voltage held, as typified in FIG. 26A, characteristic balance between the two means may be lost if characteristics of one of the two means are changed for some reason. Then the amount of current supplied to the light emitting element from the driving unit cannot be kept at a desired value any longer, thereby causing fluctuation in luminance of light emitting element between pixels.
Specifically, when μ, Co, VTH, and W/L that are characteristics unique to each TFT are deviated in the TFT 13 or 14 in FIG. 26A, the ratio of drain current of the TFT 13 to drain current of the TFT 14 varies from one pixel to another and the luminance of light emitting element fluctuates between pixels.
On the other hand, in a pixel that has means for converting a current supplied to the pixel into a voltage to hold the voltage and for causing a current to flow to a light emitting element in an amount according to the voltage held as typified in FIG. 26B, a current flows to a light emitting element when a current supplied to the pixel is converted into a voltage. The light emitting element has a relatively large capacitor. For that reason, when display is to be changed from low gray scale to high gray scale, for example, the value of voltage converted from current is not stabilized until electric charges are accumulated in the capacitor of the light emitting element. Therefore the change from low gray scale to high gray scale takes long. On the other hand, when display is to be changed from high gray scale to low gray scale, the value of voltage converted from current is not stabilized until electric charges are discharged from the capacitor of the light emitting element. Therefore the change from high gray scale to low gray scale takes long.
To be specific, it takes time for gate voltage of the TFT 33 to gain stability when the amount of current supplied from the current source 37 is changed in FIG. 26B. A time required to write the current is therefore long to bring undesirable results such as after image recognized in animation display. This cancels out a characteristic of light emitting element, suitability for animation display because of its fast response.